This invention relates to molds for forming elastomeric articles and the like, and more specifically to a vent plug for such molds.
It is desirable to prevent air from being trapped between an elastomeric article and the molding surface of a mold, such that the surface of the article may contact the molding surface uninterruptedly. If air should become trapped between the surface of the article and the molding surface, a blemish, sometimes referred to as a light spot, may result in the finished article. Examples of elastomeric articles that are formed in molds are tires and other products made of rubber or plastic.
It is a widely recognized practice to provide small bores through a mold to act as a means for venting air from between an article and the molding surface of the mold. This practice is fairly successful for eliminating trapped air, but has the disadvantage that the elastomer tends to flow into the bores resulting in undesirable projections on the outer surface of the molded article. While these undesirable projections may generally be removed from the article by an additional trimming operation, the projections are even more troublesome when they break away from the article and remain in the small bores of the mold. Naturally, if the bores become plugged, they will no longer function as vents.
If the bores through a mold body could be made small enough, very little, if any, of the elastomer being formed in the mold will flow into the bores, and any resulting projections from the surface of the molded article will be so small that they will not be easily noticed. However, because the bores are formed by drilling through the wall of a mold having a relatively large thickness, it is impractical to use a drill having a diameter of less than one to two millimeters (mm.), which still is too large to prevent undesirable projections. The present invention provides a mold with vent plugs press-fit into the bores through the mold, and the vent plugs have very small openings through them. Prior art vent plugs have diameters that interfere with the walls of the bores through a mold along substantially their entire length, so that they are difficult to remove and replace. A vent plug manufactured in accordance with this invention has an interference fit along no more than 50 percent of its overall length, so that removal and replacement is greatly facilitated when necessary. Furthermore, the plugging of the opening through a vent plug by a broken-off projection of elastomer is curtailed by slightly enlarging the opening in the vent plug, at the molding surface end, so that the base of the projection of elastomer is stronger and less likely to break when the article is removed from the mold.
These and other advantages are provided by a vent plug for a mold used in forming elastomeric articles and the like, wherein the vent plug manufactured comprises an elongated body having an axis and first and second axial ends. The overall length of the body, measured parallel to the axis, is greater than any external diameter of the body measured perpendicular to the axis. The body has at its first axial end a first external diameter which is constant over a predetermined portion of the length of the body, equal to not more than 50 percent of the overall length. The body has an external diameter along the remainder of its length that is less than said first external diameter, and an opening extends through the body from said first axial end to said second axial end.
There is provided in accordance with another aspect of the invention a mold for forming elastomeric articles and the like comprising an inner molding surface and an outer surface. The inner molding surface defines a cavity in which an article may be formed, and communicates with the outer surface by means of a bore. The bore has a vent plug therein. The vent plug comprises an elongated body having an axis and first and second axial ends. The overall length of said body, measured parallel to said axis, being greater than any external diameter of said body, measured perpendicular to said axis. The body has at its first axial end a first external diameter which is constant over a predetermined portion of the length of the body equal to not more than 50 percent of said overall length. The first external diameter is sufficiently greater than the bore so that the vent plug is press-fit into the bore. The body along the remainder of its length has an external diameter that is less than said first external diameter, and there is an opening extending through said body from said first end to said second end.